[PeterT]

Here's some info from a Kobo employee that might help:

Some information if you have experienced or heard reports of this problem:

• Some customers were experiencing an issue with Kobo eReaders (and other brands) wherein they were able to copy Adobe DRM books to their device, but not open them.
• This was due to an authorization compatibility issue introduced in Adobe Digital Editions 4. Adobe has released a new version of the software that resolves this issue.
• Customers who are currently in the bad state can upgrade to ADE 4.0.2.103411 or newer and reauthorize their device (instructions below). No need to uninstall the previous version or re-copy any books.
• Customers who authorize their device using ADE 4.0.2.103411 or newer will not experience this problem.

Notes:

• The download site still refers to Adobe Digital Editions 4.0.1, but you will actually be downloading 4.0.2.103411 or newer.
• If you need to check the version number:
  • Mac: Adobe Digital Editions menu > About Adobe Digital Editions
  • Win: Help menu > About Digital Editions

Instructions:

1. Go to http://www.adobe.com/support/digital...downloads.html to download Adobe Digital Editions 4.
2. Find and open the ADE installation file on your computer.
3. Follow the onscreen instructions to install ADE on your computer.
4. Re-authorize your Kobo eReader.
   1. Connect your eReader to your computer.
   2. Tap Connect on your eReader.
   3. Start Adobe Digital Editions.
   4. Next to Devices on the left panel, click the gear icon, then click Erase Device Authorization.
   5. In the Device Authorization window, click Erase Device Authorization, then OK.
   6. Next to Devices on the left panel, click the gear icon, then click Authorize Device.
   7. In the Device Authorization window, select your AdobeID and click Authorize Device, then OK.
   8. On your computer's desktop, right-click on KOBOeReader, and click Eject.